The present invention relates to a powder filling method and an arrangement therefor. Specifically the invention concerns a powder box arrangement for filling powder into a cavity formed in a die.
In conventional powder filling methods, powder in a powder box or filling shoe is transferred into a cavity formed in a die when the powder box is brought to a position just above the cavity. However, when entrapped air in the cavity is driven away by the powder, light particles whirl up while heavy particles drop quickly so that unevenness in particle size distribution occurs in the cavity. As a result, there is a risk that a sintered powder metallurgical component made from the powder will get a non-uniform density distribution and, as a consequence, less accurate dimensions as well as non-uniformal mechanical strength and magnetic properties.
A method eliminating some of the problems with conventional filling is disclosed in U.S. Pat. No. 5,881,357. This patent discloses a powder box having at least one pipe disposed within the powder box. Each pipe which is fastened in the wall of the powder box has a plurality of holes for supplying gas into the powder in the powder box.
An object of the present invention is to provide a powder box arrangement which eliminates the use of pipes and problems originating from the use thereof.
A second object is to provide an arrangement that can easily be adapted to suit any powder box.
A third object of the invention is to provide a powder box and a method for filling powder in a cavity uniformly and in a short time period.
A forth object of the invention is to provide a powder box and a filling method that secures good flowability of the powder in a vertical direction.
These objects as well as other objects that will be apparent from the description below, have now been obtained according to the present invention by providing a powder box according to claim 1, an arrangement for a powder box comprising an insert according to claim 4 and a method for filling powder into a cavity according to claim 8.
In brief the arrangement according to the invention concerns a powder box having arranged therein at least one insert comprising a permeable plate, a gas impermeable backing and a spacing therebetween. The spacing is connectable to a gas supply through at least one hose.
The invention also comprises a method for filling powder into a cavity formed in a die comprising the steps of:
a) supplying gas to a powder box, which is movable horizontally toward and away from a position just above the cavity, whereby the gas is supplied through a permeable plate which is arranged on at least one insert and which faces the interior of the powder box so that particles of the powder in the powder box are movable relative to each other; and
b) moving the powder box to the position just above the cavity so that the powder in the powder box enters the cavity at least by gravity.
Finally the invention also concerns the insert as defined above.
The key features of the present invention is the insert and the arrangement thereof in the powder box.
The configuration of the insert is decided by the man skilled in the art in view of the configuration of the die cavity and the powder box. Normally the insert is an essentially plane surface although other shapes may be used.
The permeable plate preferably consists of a porous material such as a polymer, metallic, or a ceramic material or a mixture thereof. The largest pore of the permeable plate should be smaller than the smallest particle of the powder to prevent the powder from clogging the pores.
Suitable materials for the backing are metals and polymers such as polyamides.
The inserts which may be prepared with an optional relation between height and length, are arranged to partially or totally cover a side wall of the powder box. The insert(s) may also be arranged so as to incline towards the bottom of the powder box or may be arranged on a cover covering the powder box. One insert is sufficient for simple cavity geoemtries.
A preferred embodiment of the powder box according to the invention comprises two inserts arranged on opposing sides of the powder box so that they are positioned on two opposing sides of the cavity when the powder box is positioned just above the cavity in the die.
The powder box is preferably covered with a cover to avoid spillage, contamination and to minimise dusting. The cover is preferably fitted with an exhaust hole to prevent the build up of the fluidising media, such as air or gas.
It can easily be realised that the insert according to the invention can be adapted to suit any powder box and any cavity configuration.
The powder filling arrangement according to the present invention improves the filling capacity by releasing air that becomes entrapped during the filling of the powder into the cavity.
Furthermore, and in contrast to the arrangement disclosed in the U.S. Pat. No. 5,881,357 which also concerns fluidised filling, the filling arrangement according to the present invention improves the filling capacity by reducing the friction of the powder particles against the walls of the powder box during the filling by using the inserts. Another advantage in comparison with the arrangement known from this U.S. patent is that the inserts are arranged in the powder box in such a way that they do not obstruct the flow of the powder during the filling of the powder into the cavity as is the case with the pipes in the known apparatus. This is especially important as every obstruction for the powder flow on the way to the cavity may cause a segregation of the powder. Unexpectedly it has also been found that approximately the same filling density can be obtained for cavities having quite different sizes and that the fill density is independent of the speed of the powder box. The arrangement for improving the filling capacity according to the present invention can also easily be adapted to a wide variety of existing powder boxes with simple fastening means, such as glue, screws, welding etc. Furthermore, the fill volume of a powder box having a given volume may be varied by adjusting the angle of inclination xcex1 which should be  greater than 0xc2x0 C. as is described below.